Power supply circuits used in computers and the like in recent years operate in a high-frequency domain of 300 kHz to 1 MHz. Inductance parts used in these power supply circuits are required to have an inductance value of about 1 μH and a DC resistance of several milliohms and also to correspond to a large current of over ten amperes. To meet these demands, there have been proposed inductance parts having dimensions of about 10 mm wide, 10 mm deep, and 4 mm high, and having a so-called compressed powder magnetic core. The compressed powder magnetic core is formed by embedding a coil in metal magnetic powder having a high saturation magnetic flux density at a high-frequency current.
A conventional inductance part will be described as follows with reference to drawings.
FIG. 14 is a perspective view of the conventional inductance part. FIG. 15 is a sectional view of FIG. 14 taken along a line 15-15. FIG. 16 is a plan view of a coil part of the conventional inductance part in which ring parts are arranged two-dimensionally. FIG. 17 is a perspective view of the coil part of the conventional inductance part in which the ring parts are superimposed on each other.
As shown in FIGS. 14 to 17, the conventional inductance part includes coil part 3, magnetic body 4, and terminals 5. The coil part is formed by bending ring parts 2 at connection parts 1 and superimposing them over each other. Ring parts 2 are made of metal plates arranged two-dimensionally and connected via connection parts 1. Magnetic body 4 is formed by pressure-molding a mixture of insulating resin and metal magnetic powder in such a manner that coil part 3 is embedded in the mixture. Terminals 5 are formed by extending both ends of coil part 3 from magnetic body 4. Ring parts 2 are provided on their surfaces with insulating film 6 (shown with a hatched pattern in FIG. 16). The insulating film prevents a short-circuit between adjacent ring parts 2 when they are superimposed on each other. An example of a conventional technique related to this application is shown in Patent Literature 1.
To achieve further miniaturization of electronic devices, power supply circuits have been miniaturized by increasing the operating frequency in recent years. In line with this, inductance parts have also been strongly required to operate at higher frequencies and to reduce their sizes. In order to miniaturize this type of inductance parts, it is necessary to increase the relative magnetic permeability of magnetic body 4, for which it is necessary to increase the filling factor of the metal magnetic powder in magnetic body 4.
In the above-described conventional inductance part, insulating film 6 is provided on the surface of each of ring parts 2 so as to prevent a short-circuit between adjacent ring parts 2 when they are superimposed on each other. Therefore, it is essential, during the formation of magnetic body 4 by pressure-molding, to prevent insulating film 6 from being damaged by ring parts 2 that have broken into each other due to the deformation of coil part 3 or being broken by the metal magnetic powder. To achieve this, magnetic body 4 is formed by pressure-molding metal magnetic powder at a pressure of 196 to 392 MPa (2 to 4 t/cm2). Under this pressure, however, the filling factor of the metal magnetic powder in magnetic body 4 cannot be set higher than 75% by volume. Therefore, the conventional method has limitations in miniaturizing inductance parts by increasing the filling factor of the metal magnetic powder in magnetic body 4.    Patent Literature 1: Japanese Patent Unexamined Publication No. 2004-111456